Soluble triarylamine functionalized symmetric viologen for all-solid-state electrochromic supercapacitors

Article abstract of DOI:10.1007/s11426-020-9789-9

Electrochromic supercapacitors have drawn enormous attention due to their ability to monitor the charge and discharge processes through color changes of electroactive materials. However, there are few work on small organic molecules as active materials fo

Full text of DOI:10.1007/s11426-020-9789-9

SCIENCE CHINA
Chemistry
•ARTICLES•
https://doi.org/10.1007/s11426-020-9789-9
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Soluble triarylamine functionalized symmetric viologen for
all-solid-state electrochromic supercapacitors
Yanling Zhuang1, Weiwei Zhao1, Longlu Wang1, Feiyang Li1, Weikang Wang1, Shujuan Liu1*,
Wei Huang1,2 & Qiang Zhao1*
1 Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials,
Nanjing University of Posts and Telecommunications, Nanjing 210023, China;
2
Frontiers Science Center for Flexible Electronics & Shaanxi Institute of Flexible Electronics, Northwestern Polytechnical University,
Xi’an 710072, China
Received April 7, 2020; accepted June 8, 2020; published online August 13, 2020
Electrochromic supercapacitors have drawn enormous attention due to their ability to monitor the charge and discharge processes
through color changes of electroactive materials. However, there are few work on small organic molecules as active materials for
all-solid-state electrochromic supercapacitors. Herein, we reported two novel multifunctional symmetric viologens (TPA-bpy
and CZ-bpy), which showed different solvatochromic, electrochromic, electroluminochromic and energy storage behaviors
despite their similar chemical structures. The different performances between these two viologens were attributed to the
difference in the intramolecular charge transfer capability and the solubility in organic solvents. Devices containing TPA-bpy
displayed faster response time and higher coloration efficiency due to the introduction of packing-disruptive and three-di-
mensional triarylamine groups. Moreover, devices containing TPA-bpy also showed energy storage characteristics with an
obvious color change from purple to yellow. It showed a wide voltage window (2.0 V), long discharge time (230.3 s at
0.01 mA cm−2), and excellent cycling stability with 90% capacitance retention after 6,000 cycles. The work provides a new and
convenient strategy towards the development of novel electrochromic capacitive materials.
viologen, solvatochromism, electrochromism, electroluminochromism, supercapacitors
Citation:
Zhuang Y, Zhao W, Wang L, Li F, Wang W, Liu S, Huang W, Zhao Q. Soluble triarylamine functionalized symmetric viologen for all-solid-state
electrochromic supercapacitors. Sci China Chem, 2020, 63, https://doi.org/10.1007/s11426-020-9789-9
1 Introduction
Therefore, the energy storage/release process can be quan-
titatively monitored by the optical transmittance and in-
dicated by a color change [15]. These unique properties
make all-solid-state electrochromic supercapacitors suitable
for practical applications in cars, airplanes and buildings
[10].
To date, research on electrochromic capacitive materials
has mainly focused on transition metal oxides and hydro-
xides [16–18]. The slow switching time and poor coloring
efficiency hinder the prosperous development of inorganic
electrochromic supercapacitive materials [19]. Organic
electrochromic supercapacitive materials, particularly con-
All-solid-state electrochromic supercapacitors have attracted
immense research interests due to their ability to meet the
needs of small and wearable electronic devices [1–5] and
their combined performances of optical modulation and en-
ergy storage [6–10]. The optical properties can be reversibly
changed by an external voltage, and the charge/discharge
process is accompanied by color changes, which are de-
pendent on different redox states (faradic processes) [11–14].
*Corresponding authors (email: iamsjliu@njupt.edu.cn; iamqzhao@njupt.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
chem.scichina.com link.springer.com

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Zhuang et al. Sci China Chem
ductive polymers, have also attracted great research interest
due to their excellent properties, such as easy structure
modification, rich redox states, wide voltage window and
high energy density. However, small organic molecules are
rarely reported as electrochromic supercapacitive materials
because they are difficult to simultaneously exhibit electro-
chromic and energy storage properties. Dicationic qua-
ternary salts of 4,4′-bipyridine, also known as viologen, have
been one of the most extensively studied electrochromic,
electroluminochromic or energy storage materials due to
their excellent electron-accepting capability, rich redox
states, and easy tunability of the substituents [20–28].
Viologen-based electrochromic materials including small
molecules, polymers and composite materials have been
extensively studied [29,30]. For small molecular viologens,
the color and spectral characteristics are modulated by
changing the substituents on the nitrogen atoms. Conjugated
polymers are obtained by introducing viologen units into the
main chain or side chain, while polyviologens can be pre-
pared by the reduction electropolymerization. Viologen-
based composite materials can be acquired in a variety of
ways, such as introducing viologen groups onto various
nanostructured semiconductors, mixing with carbon nano-
materials or self-assembling through two or more viologens.
Compared with polymers and composite materials, small
molecular viologens are easier to be synthesized and their
photophysical properties are easier to be regulated. There-
fore, rich color variations are easier to be obtained. Under
electrical stimulus, the fluorescence change for electro-
luminochromic materials based on viologen is achieved by
inserting aromatic groups between two pyridine groups, such
as dithiophene, thiazolothiazole, benzene, naphthalene, an-
thracene and benzothiadiazole moieties [31]. However, only
two kinds of fluorescence were achieved before and after the
voltages are applied. For capacitors, viologens are often used
as active materials together with other capacitive materials,
and their capacity characteristics are rarely explored [29].
Therefore, small molecular viologens are highly desirable as
the promising candidate for electrochromic, electro-
luminochromic and supercapacitive materials.
contrast and high coloration efficiency. Importantly, all-so-
lid-state electrochromic supercapacitors was also fabricated
using TPA-bpy as the electroactive materials. The super-
capacitors displayed a wide voltage window, long discharge
time and excellent charging/discharging cyclic stability. To
the best of our knowledge, this is the first example about
small organic molecules for all-solid-state electrochromic
supercapacitors.
2 Experimental
The details of general experimental sections were provided
in the Supporting Information online.
2.1 Synthesis and characterization of 4
The mixture of 2 (0.18 mmol, 100 mg) and 3 (0.80 mmol,
299 mg) in solution of ethanol/deionized water (1:1, v/v,
60 mL) was refluxed for 84 h under nitrogen atmosphere.
After the mixture was cooled to room temperature, the so-
lution was filtered to remove the insoluble matters, and the
filtrate was evaporated to dryness under reduced pressure.
Recrystallization from methanol/ether to give a purple solid.
1
Yields: 115 mg (68.0%). H nuclear magnetic resonance,
1H NMR (400 MHz, MeOH-d4) δ (ppm): 9.46 (d, J=6.8 Hz,
4H), 8.84 (d, J=6.4 Hz, 4H), 7.72 (d, J=8.8 Hz, 4H), 7.48 (d,
J=8.4 Hz, 4H), 7.17 (m, 12H), 1.37 (s, 36H). Electrospray
ionization high-resolution mass spectrometry, ESI-HRMS
(m/z): [M]+ Calcd. for C62H68N4: 868.54330, found:
434.27115. Elemental analysis: Anal. Calcd. for C62H68N4Cl2:
C, 79.21; H, 7.29; N, 5.96. Found: C, 79.72; H, 7.15; N,
5.87%.
2.2 Synthesis and characterization of TPA-bpy
The mixture of 4 (0.12 mmol, 115 mg) and ammonium
hexafluorophosphate (NH4PF6) (1.00 mmol, 163 mg) in so-
lution of ethanol/deionized water (4:1, v/v, 60 mL) was
stirred for 12 h at room temperature. A purple solid was
obtained after filtration and recrystallization from methanol/
In this work, we have designed and synthesized two novel
symmetric viologens (TPA-bpy and CZ-bpy) by in-
corporating electron-rich triphenylamine or carbazole moi-
ety into 4,4′-bipyridine to be used as all-solid-state
electrochromic supercapacitor materials. Although both
viologens exhibited rich redox states, tunable absorption and
emission properties, and interesting electrochromic and
electroluminochromic performance, TPA-bpy had better
performance due to the introduction of damaging packaging
and three-dimensional triarylamine groups. Furthermore,
TPA-bpy or CZ-bpy-containing all-solid-state electro-
chromic and electroluminochromic devices had excellent
performance, such as rapid response time, good optical
1
ether. Yields: 73 mg (41.0%) H NMR (400 MHz, DMSO-
d6) δ (ppm): 9.57 (d, J=6.4 Hz, 4H), 8.96 (d, J=7.2 Hz, 4H),
7.77 (d, J=8.8 Hz, 4H), 7.47 (d, J=8.4 Hz, 8H), 7.13 (d, J=
8.8 Hz,
8H),
7.05
(d,
J=8.8 Hz,
4H), 1.31 (s, 36H). 13C NMR (100 MHz, DMSO-d6) δ
(ppm): 150.9, 148.3, 148.2, 145.3, 143.6, 134.4, 127.3,
126.9, 126.1, 126.0, 119.8, 34.5, 31.6. 19F NMR (377 MHz,
DMSO-d6) δ (ppm): −70.13 (d, J=711.6 Hz). ESI-HRMS
(m/z): [M]+ Calcd. for C62H68N4: 868.54330, found:
434.27121. Elemental analysis (%): Anal. Calcd. for
C62H68N4P2F12: C, 64.24; H, 5.91; N, 4.83. Found: C, 64.40;
H, 6.05; N, 4.91.

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Zhuang et al. Sci China Chem
2.3 Synthesis and characterization of 7
supercapacitors. The film thickness of the devices was
measured by the scanning electron microscope (SEM) and
the average film thickness is 17.97 µm. Cross-sectional SEM
images of the devices are shown in Figure S1 (Supporting
Information online).
The mixture of 3 (0.18 mmol, 100 mg) and 6 (0.80 mmol,
297 mg) in solution of ethanol/deionized water (5:1, v/v,
60 mL) was refluxed under nitrogen atmosphere for 84 h.
After the mixture was cooled to room temperature, the so-
lution was filtered to remove the insoluble matters, and the
filtrate was evaporated to dryness under reduced pressure.
Recrystallization from methanol/tetrahydrofuran to obtain a
2.6 Related calculations for all-solid-state super-
capacitors
1
red solid. Yields: 101 mg (60.0%). H NMR (400 MHz,
The specific capacitance (C) is calculated from the galva-
MeOH-d4) δ (ppm): 9.84–9.70 (m, 4H), 9.13–8.99 (m, 4H),
8.34–8.20 (m, 8H), 8.12 (d, J=8.0 Hz, 4H), 7.63–7.52 (m,
8H), 1.51 (s, 36H). ESI-HRMS (m/z): [M]+ Calcd. for
C62H64N4: 868.51200, found: 864.50952. Elemental analysis
(%): Anal. Calcd. for C62H64N4Cl2: C, 79.55; H, 6.89; N,
5.99. Found: C, 78.83; H, 6.88; N, 6.73.
nostatic charge/discharge (GCD) curves according to the
following equation:
It
C =
(1)
V
The energy density (E) and power density (P) are calcu-
lated from the following equations, respectively:
1
1
E = × C × ( V)2 ×
(2)
2
3600
2.4 Synthesis and characterization of CZ-bpy
E
P = 3600 ×
(3)
The mixture of 7 (0.11 mmol, 101 mg) and NH4PF6
(1.00 mmol, 163 mg) in solution of ethanol/deionized water
(5:1, v/v, 60 mL) was stirred for 12 h at room temperature. A
fuchsia solid was obtained after filtration and washing with
ethanol and water. Yields: 87 mg (55%). 1H NMR
(400 MHz, DMSO-d6) δ (ppm): 9.87 (d, J=6.4 Hz, 4H), 9.19
(d, J=6.8 Hz, 4H), 8.41–8.36 (m, 4H), 8.28 (d, J=8.8 Hz,
4H), 8.16 (d, J=8.4 Hz, 4H), 7.58 (d, J=8.8 Hz, 4H), 7.48 (d,
J=8.8 Hz, 4H), 1.46 (s, 36H). 13C NMR (100 MHz, DMSO-
d6) δ (ppm): 149.6, 146.1, 144.0, 140.8, 140.3, 138.3, 127.9,
127.2, 127.1, 124.5, 123.8, 117.3, 109.5, 35.0, 32.0. 19F
NMR (377 MHz, DMSO-d6) δ (ppm): −70.12 (d, J=
711.6 Hz). ESI-HRMS (m/z): [M]+ Calcd. for C62H64N4:
868.51200, found: 864.50958. Elemental analysis (%): Anal.
Calcd. for C62H64N4P2F12: C, 64.47; H, 5.58; N, 4.85. Found:
C, 63.35; H, 5.57; N, 4.92.
t
where I is the applied current, t is the discharge time, ΔV is
the actual potential window excluding the IR drop.
3 Results and discussion
3.1 Synthesis and characterization of viologen deriva-
tives
The synthetic procedures of TPA-bpy and CZ-bpy are shown
in Scheme 1 in three steps. Firstly, the intermediate 3 was
obtained by the Zincke reaction between 1-chloro-2,4-dini-
trobenzene and 4,4′-bipyridine. Next, 4 and 7 were respec-
tively synthesized by refluxing the intermediate 3 and the
corresponding aromatic amine 2 or 6 in a mixed solvent of
ethanol and water for 84 h under nitrogen atmosphere. At
last, 4 and 7 were respectively stirred with NH4PF6 in a
mixed solvent of water and ethanol at room temperature for
12 h to obtain target products TPA-bpy and CZ-bpy. Among
them, compounds 1–3, 5 and 6 were synthesized according to
the literature reports [32–34]. The new intermediates and
viologen derivatives were characterized by NMR (1H NMR,
13C{1H} NMR and 19F{1H} NMR), ESI-HRMS and ele-
mental analysis. The purpose of introducing a large tert-butyl
substituent at the active site of the triphenylamine and car-
bazole units was to increase the solubility and avoid elec-
trochemical polymerization of the viologens. Ion exchange
was used to further improve the solubility of TPA-bpy and
CZ-bpy in organic solvents.
2.5 Fabrication of all-solid-state electrochromic and
electroluminochromic devices and supercapacitors
Electrochromic and electroluminochromic film was prepared
by dropping the TPA-bpy or CZ-bpy and polyvinylpyrro-
lidone (PVP, film-former, mass ratio=1:1) in acetonitrile
solution onto indium-doped tin oxide (ITO)-coated glass.
The film was vacuum dried at 80 °C for 24 h for the com-
plete curing. A transparent gel electrolyte was obtained by
mixing polymethyl methacrylate (PMMA, Mw: 120,000,
0.7 g), LiClO4 (0.3 g) and propylene carbonate (PC, 3.0 g) in
dry acetonitrile (2 mL). The mixture was stirred rapidly at
50 °C until a gelation occurred. The gel electrolyte was
coated on a new ITO-coated glass by a pulling method. The
above two ITO-coated glasses were bonded together and
dried in a vacuum oven at 80 °C for 12 h to obtain all-solid-
state electrochromic and electroluminochromic devices and
3.2 Photophysical and electrochemical properties of
viologen derivatives
The optical properties of TPA-bpy and CZ-bpy were studied

4
Zhuang et al. Sci China Chem
Scheme 1 Synthetic routes of TPA-bpy and CZ-bpy.
by ultraviolet-visible (UV-Vis) absorption spectroscopy as
shown in Figure 1(a). Both TPA-bpy and CZ-bpy have two
absorption peaks. Among them, the absorption peak around
300 nm is the characteristic absorption of viologen, and the
maximum absorption wavelength around 500 nm can be at-
tributed to charge transfer transition from triphenylamine or
carbazole moieties to 4,4′-bipyridine. In addition, the max-
imum absorption peak of TPA-bpy is red shifted by about
40 nm from that of CZ-bpy. This change in absorption is
accompanied by an increase in the energy gap from 1.80 eV
(TPA-bpy) to 2.03 eV (CZ-bpy) and can be explained by the
stronger electron donating ability of the triphenylamine than
that of the carbazole group (Table S1, Supporting Informa-
tion online). Figure 1(b) exhibits the emission spectra of
TPA-bpy and CZ-bpy. The maximum emission peaks of
TPA-bpy and CZ-bpy are about 600 and 425 nm, respec-
tively. The emission wavelength of TPA-bpy is significantly
red-shifted compared with that of CZ-bpy, which indicates
that the intramolecular charge transfer capability of TPA-bpy
is greater than that of CZ-bpy. This experimental phenomena
can be further confirmed by theoretical calculations. The
frontier orbital distributions of TPA-bpy and CZ-bpy are

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Zhuang et al. Sci China Chem
Figure 1 (a) UV-Vis absorption and (b) emission spectra of TPA-bpy and CZ-bpy (1×10−5 M) in dry acetonitrile at 298 K. Photographs of (c) TPA-bpy and
(d) CZ-bpy (2.5×10−4 M) in different solvents. UV-Vis absorption spectra of (e) TPA-bpy and (f) CZ-bpy (1×10−5 M) in different solvents at 298 K. Emission
spectra of (g) TPA-bpy and (h) CZ-bpy (1×10−5 M) in different solvents at 298 K (color online).
shown in Section 3.3. The highest occupied molecular orbital
(HOMO) of the two viologen derivatives are distributed on
the triphenylamine or carbazole groups, respectively, while
the lowest unoccupied molecular orbital (LUMO) are mainly
distributed on the bipyridyl group and the benzene rings at-
tached to the bipyridine, indicating the possible in-
tramolecular charge transfer transition.
Triphenylamine and carbazole groups display excellent
hole-transport properties, and are typically incorporated into
electrochromic materials because lone electron pairs of the
nitrogen atoms on them can be effectively oxidized to a
cationic free radical state, which is usually accompanied by

6
Zhuang et al. Sci China Chem
the color change [34–38]. Moreover, triphenylamine moiety
can prevent intermolecular aggregation due to its non-planar
structure such that the solubility and film-forming property
of TPA-bpy can be improved [34–37]. Therefore, the sol-
vatochromic, electrochromic, electroluminochromic and
energy storage behaviors of TPA-bpy are superior to those of
CZ-bpy.
confirmed by the reported literatures [23–25,34–37]. The
two pairs of redox peaks obtained during the cathode scan
can be attributed to the two one-electron redox processes of
the 4,4′-bipyridine units, corresponding to the generation of
radical cationic and neutral species, respectively. The for-
mation of bi-radical cationic substances during oxidation of
the triphenylamine and carbazole units resulted in redox
waves observed by TPA-bpy and CZ-bpy during the anode
scan. Interestingly, compared to CZ-bpy (E1/2Red1=−0.47 V,
Interestingly, both TPA-bpy and CZ-bpy showed solvato-
chromic behaviors, but TPA-bpy has better performance due
to the introduction of damaging pack aging and three-
dimensional triarylamine groups. In order to better demon-
strate the solvatochromic properties, TPA-bpy and CZ-bpy
were first studied in various solvents with different pola-
rities. The colors of TPA-bpy in different solvents were light
pink, purple, violet and blue, while CZ-bpy in different
solvents only showed orange and light pink (Figure 1(c, d)).
The UV-Vis absorption spectra and absorption maxima of
two viologen derivatives in different solvents are shown in
Figure 1(e, f) and Table S2, respectively. The maximum ab-
sorption wavelengths of TPA-bpy are ranged from 425 to
572 nm, and those of CZ-bpy are about 438–535 nm. In
addition, the emission spectra of TPA-bpy and CZ-bpy in
different solvents were further investigated as shown in
Figure 1(g, h). All emission spectra for TPA-bpy are similar
without significant red- or blue-shift, while CZ-bpy in other
solvents has two emission peaks except for acetone and
acetonitrile and the emission peaks mainly centered at
408–435 nm, indicating no correlation between emission
spectra and solvent polarity. The solvatochromism of TPA-
bpy and CZ-bpy indicates that the solvents have a greater
influence on the ground state dipole moment and little effect
on the excited-state dipole moment [39,40].
E1/2Red2=−0.76 V, E1/2Ox=1.10 V), TPA-bpy has lower redox
Ox
potentials (E1/2Red1=−0.57 V, E1/2Red2=−0.91 V, E1/2
=
0.75 V). This can be explained by the fact that lone electron
pairs of the nitrogen atoms on carbazole groups are pre-
ferably located in the coplanar ring, such that the oxidation
potential of the carbazole moiety is higher than that of the
triphenylamine group, resulting in more difficult reduction of
the bipyridyl group on CZ-bpy [37].
3.3 Theoretical calculations
Density functional theory (DFT) calculations have been
performed for TPA-bpy and CZ-bpy in order to further
confirm the experimental findings (Figure 3). The calculated
results are in consistent with the trends observed by UV-Vis
absorption spectra and CV curves (Table S1). Two viologen
derivatives exhibit similar and clearly visible HOMO and
LUMO distributions. Because of the donor-character of the
triphenylamine or carbazole substituents, the HOMOs are
mainly located in the donor parts of the viologens. The
LUMOs are corresponding to varying degrees of delocali-
zation on nitrogen atom of triphenylamine or carbazole
groups and the N-aryl substituents. For the radical cationic
states, the LUMOs clearly show electron density contribu-
tion from the N-aryl substituents and HOMOs are distributed
on the entire π*-system, which indicates a strong delocali-
zation of the molecules. Poor delocalization is exhibited for
the neutral states because their HOMOs and LUMOs are
mainly distributed on the N-aryl substituents and the benzene
rings attached to the N-aryl substituents. However, the LU-
MOs and HOMOs for bi-radical cationic state are still mainly
The cyclic voltammetry (CV) curves of TPA-bpy and CZ-
bpy are shown in Figure 2(a, b), and the electrochemical data
are summarized in Table S2. Both TPA-bpy and CZ-bpy
underwent reversible two-electron reduction and single-
electron oxidation. Scheme S1 (Supporting Information on-
line) demonstrates the proposed reduction and oxidation
reaction sequence for TPA-bpy and CZ-bpy for producing
different anionic and cationic substances, which can be
Figure 2 Cyclic voltammetry curves of (a) TPA-bpy, (b) CZ-bpy recorded in anhydrous acetonitrile (0.1 M nBu4NPF6) at 298 K using a glassy carbon
working electrode at a sweep rate of 100 mV s−1 (color online).

7
Zhuang et al. Sci China Chem
Figure 3 HOMO and LUMO distributions of TPA-bpy and CZ-bpy (color online).
distributed on the N-aryl substituents and triphenylamine or
carbazole groups, respectively. The different electron density
distributions and intramolecular charge transfer capability
exhibited by the different oxidation states of the two mole-
cules result in their rich color and luminescence changes
under electrical stimulus.
sion. After 0.1 eq. of NaBH4 was added, both the solution
and luminescence colors changed to yellow, indicating that
TPA-bpy has been reduced to radical cationic species. When
0.2 eq. of NaBH4 was added, both the solution and
luminescence colors changed to pale yellow and blue, re-
spectively, indicating that TPA-bpy was further reduced to
neutral species. There was no significant change in the color,
and only the luminescence intensity increased after the ex-
cess NaBH4 was dropped (Figure 4(a)). The UV-Vis ab-
sorption and emission spectra of TPA-bpy with the addition
of NaBH4 are shown in Figure 4(b, c). Unlike TPA-bpy, the
solution and luminescence colors of CZ-bpy changed
slightly when NaBH4 was added. Moreover, when 0.2 eq. of
NaBH4 was added, the solution color changed from orange to
yellow, and the luminescent color did not change sig-
nificantly, but the intensity decreased. The solution became
light yellow and the luminescent intensity was enhanced by
adding 0.4 eq. of NaBH4. The solution changed from light
yellow to colorless, and the emission intensity continued to
3.4 Chemical reduction of viologen derivatives
It is recognized that electrochromism was due to the reduc-
tion of 4,4′-bipyridine and the oxidation of triphenylamine or
carbazole groups in two viologens [37,41,42]. Therefore, the
discoloration and luminescence responses of TPA-bpy and
CZ-bpy towards different reducing agents including NaBH4,
NaHS, KI, and KBr have been studied. Interestingly,
TPA-bpy and CZ-bpy underwent color and luminescence
change only in the presence of the reducing agent NaBH4 at
room temperature. Before NaBH4 was added, the TPA-bpy
acetonitrile solution was purple and showed orange emis-

8
Zhuang et al. Sci China Chem
Figure 4 (a) Photograph of TPA-bpy (2.5×10−4 M) with the addition of NaBH4 in anhydrous acetonitrile at 298 K. (b) UV-Vis absorption and (c) emission
spectra of TPA-bpy (1×10−5 M) with the addition of NaBH4 in anhydrous acetonitrile at 298 K. (d) Photographs of CZ-bpy (2.5×10−4 M) with the addition of
NaBH4 in anhydrous acetonitrile at 298 K. (e) UV-Vis absorption and (f) emission spectra of CZ-bpy (1×10−5 M) with the addition of NaBH4 in anhydrous
acetonitrile at 298 K (color online).
increase when an excess of NaBH4 was dropped (Figure 4
(d)). This change process can be proven by the UV-Vis ab-
sorption and emission spectra of CZ-bpy with the addition of
NaBH4 (Figure 4(e, f)). The above results indicate that TPA-
bpy is more easily reduced by NaBH4 than CZ-bpy. Fluor-
escence quantum yields (Φ) of two viologen with different
redox states in anhydrous acetonitrile at 298 K are shown in
Table S3.
3.5 Fabrication and performance of all-solid-state
electrochromic and electroluminochromic devices
On the basis of the above results, we have successfully
prepared the bi-functional electrochromic and electro-
luminochromic devices using TPA-bpy or CZ-bpy as elec-
troactive materials, respectively. The schematic diagram of
the devices is shown in Figure S2. For the TPA-bpy-con-

9
Zhuang et al. Sci China Chem
taining devices, the film was purple and displayed orange
emission before the external voltage was applied. Both the
film and luminescence colors gradually turned to yellow due
to the formation of radical cationic species when a voltage of
−1.2 V was applied (Figure 5(a)). Furthermore, due to the
accumulation of neutral substances, the film and the lumi-
nescent colors changed to pale yellow and blue, respectively,
by applying a higher voltage of −1.6 V (Figure 5(a)). In
addition, the yellow film and blue emission were observed at
an external electrical bias of 2.0 V (Figure 5(a)). The cor-
responding UV-Vis absorption and emission spectra of the
TPA-bpy-containing bi-functional devices at different vol-
tages were further explored (Figure 5(b–e)). Prior to the
voltage applied, two absorption peaks were observed for
TPA-bpy-containing devices. The absorption peaks around
300 and 500 nm had a red shift of about 40 nm and slight
blue shift, respectively, compared to those in acetonitrile. At
an external potential bias of −1.2 V, two absorption peaks
had slightly red and blue shifts, respectively. The absorption
peak around 300 nm was still red-shifted, while that at
500 nm disappeared at a voltage of −1.6 or 2.0 V. The
emission spectra of the devices under different negative
pressures are similar to those in the above chemical reduction
experiment.
The UV-Vis transmittance spectra of the TPA-bpy-
containing bi-functional devices under different applied
voltages are shown in Figure 5(f, g). At the voltages of −1.2
and −1.6 V, the TPA-bpy film had optical transmittance
changes (∆T%) of 46% and 52% at 411 nm, respectively,
while a higher optical contrast (76%) was obtained at 380 nm
at an external potential bias of 2.0 V. Figure 6(a) describes
the optical transmittance at 411 nm as a function of time by
applying square-wave potential steps between 0 and −1.2 V
with a resident time of 20 s for the TPA-bpy film. The color
switching time is defined as the time required to reach 90%
of the total change in transmittance after switching the po-
tential, because it is difficult to visually observe any further
color changes after this point [35–37,42]. The coloring and
bleaching times were 4.5 and 2.2 s, respectively. (Figure 6
(b)). Figure S5a displays a very small change in ΔT% as the
potential square wave resonance number increases, which
indicated that the TPA-bpy film has high stability upon re-
duction to radical cationic species. When the voltage was set
between 0 and −1.6 V, the coloring and bleaching times of
Figure 5 (a) Photographs, (b, c) UV-Vis absorption, (d, e) emission and (f, g) transmittance spectra of the TPA-bpy-containing bi-functional devices under
different voltages (color online).

10
Zhuang et al. Sci China Chem
Figure 6 (a, c, e) Current consumption and (b, d, f) electrochromic/electroluminochromic switching studies for the TPA-bpy-containing bi-functional
devices monitored at λmax=411, 411 and 380 nm, respectively, at external applied potentials with a residence time of 20 s (color online).
the devices were 4.0 and 2.8 s, respectively (Figure 6(c, d)).
However, ΔT% had a large loss after 1,600 s as the number
of potential square wave cycles increased, which meant that
the reduced stability was exhibited in the neutral state of
the TPA-bpy film (Figure S5(a)). The bi-functional devices
required 5.5 s for coloring and 3.5 s for bleaching by
applying square-wave potential steps between 0 and 2.0 V
with a resident time of 20 s at 380 nm (Figure 6(e, f)).
Moreover, ∆T% hardly changed as the number of potential
square wave cycles increased, which also represented that
the TPA-bpy film had high stability upon oxidation to bi-
radical cationic species. Coloration efficiency (CE, η) can be
calculated by the equation of η=∆OD/Q, which is a mea-
surement of the power efficiency of a dual-functional device,
where Q is the injected/extracted charge per unit electrode
area (mC cm−2) and obtained by integrating Figure 6(a, c, e),
and ΔOD is the change in absorbance at a specific wave-
length maximum at different redox states [35–37,42]. The
high CE values of radical cationic state, neutral state and bi-
radical cationic state of the devices containing TPA-bpy were
calculated to be 171, 250 and 294 cm2 C−1, respectively
(Table S4).
before an electrical bias was applied. When the radical ca-
tionic species was formed at the voltage of −1.6 V, the film
color changed from orange to yellow, whereas the lumines-
cence did not change significantly, but the intensity was
slightly reduced by the naked eye (Figure S3(a)). Further
increasing the potential to −2.0 V, both the film and lumi-
nescence colors did not change much, but the luminescence
intensity increased by the naked eye, indicating the forma-
tion of neutral substances (Figure S3(a)). When the applied
voltage was 2.4 V, the film changed from orange to yellow
and the luminescence was still blue, but the luminescence
intensity increased (Figure S3(a)). The UV-Vis absorption
and emission spectra of the CZ-bpy-containing bi-functional
devices at different voltages had also been studied (Figure S3
(b–e)). Two absorption peaks at about 400 and 600 nm were
obtained for the CZ-bpy-containing devices prior to the
voltage was applied, and both showed a red shift of about
100 nm compared to the corresponding peak in the acetoni-
trile solution. At −1.6 V, the peaks had a blue shift of ap-
proximately 20 and 50 nm, respectively. However, at an
external electrical bias of −2.0 V, the absorption peak around
400 and 600 nm continued to be blue-shifted and dis-
appeared, respectively. The peaks had a blue shift of ap-
proximately 15 and 100 nm, respectively, at a voltage of
2.4 V. The emission spectra of the CZ-bpy-containing de-
vices at different voltages were consistent with those of
chemical reduction experiment.
For the CZ-bpy-containing bi-functional devices, both film
and the luminescent color changes can also be achieved
under different applied voltages, but the change in lumi-
nescent colors were not as rich as that of the TPA-bpy-con-
taining devices. Moreover, the CZ-bpy-containing devices
needed a higher applied voltage when the same redox state as
the TPA-bpy-containing devices was obtained. More speci-
fically, the film was orange and exhibited blue emission
The corresponding UV-Vis transmittance spectra of the
CZ-bpy-containing devices at different voltages are shown in
Figure S3(f, g). The film had a high optical contrast of up to

11
Zhuang et al. Sci China Chem
55% at 376 nm at −1.6 V, 79% at 365 nm at −2.0 V, and 64%
at 384 nm at 2.4 V, respectively. The optical transmittance at
376, 365 and 384 nm as a function of time was studied by
applying square-wave potential steps between 0 and −1.6 V,
0 and −2.0 V, 0 and 2.4 V, respectively, with a resident time
of 20 s for the CZ-bpy film (Figure S4). The three different
redox states of the CZ-bpy film acquired coloring times of
5.5, 16.6 and 5.8 s, respectively, and the fading times were
2.7, 9.3 and 3.0 s, respectively. The electrochromic and
electroluminochromic switching studies of the three redox
states of the CZ-bpy film are shown in Figure S5(b). As the
number of potential square wave cycles increased to 100
cycles, ∆T% in 0–2.4 V had slight fluctuation, and that in
0–−1.6 V and 0–−2.0 V repeated well, indicating that the
CZ-bpy film had excellent stability in the reduction to radical
cationic and neutral species. The CE values of radical ca-
tionic, neutral and bi-radical cationic states of the devices
containing CZ-bpy were calculated to be 151, 216 and
206 cm2 C−1, respectively (Table S4). The reasons for the
difference in performance between the two bi-functional
devices may be as follows: (1) TPA-bpy has a higher in-
tramolecular charge transfer capability than CZ-bpy, so the
devices containing TPA-bpy have lower driving voltages; (2)
introducing packing-disruptive and three-dimensional triar-
ylamine groups into 4,4′-bipyridine not only resulted in the
enhancement of solubility (Table S5), but also avoided
concentration quenching effectively due to π-π interactions
among TPA-bpy molecules, which makes TPA-bpy exhibit
richer color changes under electrical stimulus [35–37].
3.6 Fabrication and performance of all-solid-state
electrochromic supercapacitors
Based on the above research on electrochromic and elec-
troluminochromic bi-functional devices, the devices con-
taining TPA-bpy or CZ-bpy may be referred to as pseudo-
capacitors. The study found that the devices containing TPA-
bpy had pesudocapacitive performance, while the CZ-bpy
film did not have constant current charge and discharge
properties, which were attributed to its poor solubility and
film formation properties resulted from its more planar
structure [35–37]. For the TPA-bpy-based supercapacitors,
the CV curves exhibited two reversible redox processes at
different scan rates (5 to 200 mV s−1, Figure 7(a)) from 0 to
2.0 V, which were related to the reversible two electron gain
and loss processes of the acceptor 4,4′-bipyridine [43]. As
the scan rate increased, the oxidation and reduction peaks
moved to the more positive and negative positions of the
potential, respectively. Moreover, the well-defined redox
peaks were still maintained even at the high scanning rate of
200 mV s−1, indicating that the supercapacitors had low in-
ternal resistance, good electrochemical capacitive nature and
fast charge transfer [44]. The pseudocapacitive behaviors of
the devices containing TPA-bpy were further investigated by
GCD curves (Figure 7(b)) at various current densities, in
which the GCD curves were nonlinear, indicating the pre-
sence of redox reactions from TPA-bpy. The upward and
downward lines correspond to the charging and discharging
processes, respectively, which were related to the mutual
Figure 7 (a) CV curves of the supercapacitors containing TPA-bpy under different scan rates in the potential range of 0–2.0 V. (b) GCD curves of the
supercapacitors containing TPA-bpy collected at various current densities. (c) GCD curves at 0.01 mA cm–2 in the potential range of 0–2.0 V and corre-
sponding in-situ transmittance change measured at 411 nm for the supercapacitors containing TPA-bpy. Inset: photographs of the supercapacitors containing
TPA-bpy under various energy storage levels, 0 and 2.0 V. (d) Areal capacitance obtained from GCD curves. (e) Energy and power density of the solid-state
supercapacitors with a comparison with the previously reported devices. (f) Cycling stability of the supercapacitors at 0.04 mA cm−2. Inset: the firstand last 5
cycles of the supercapacitors (color online).

12
Zhuang et al. Sci China Chem
conversion between different redox states in the TPA-bpy
film. As TPA-bpy film simultaneously functions in the
electrochromic and energy storage applications and shares
the same electrochemical process in the same electrolyte, the
GCD curve at 0.01 mA cm−2 and the corresponding in situ
transmittance change at 411 nm were explored (Figure 7(c)).
During the charging process, TPA-bpy was first reduced to
radical cationic species and then reduced to neutral sub-
stances with the film color changing from purple to yellow
(Figure 7(c)). The fully charged state was obtained at 2.0 V,
and the film was still yellow. The film gradually turned to be
purple during the discharging process. The electrochromic
supercapacitors exhibited an optical contrast of ~50% be-
tween the charged (2.0 V) and discharged (0 V) states at
411 nm. The areal capacitances (C) reached 1.25 mF cm−2 at
0.01 mA cm−2 (Figure 7(d)), which is higher than electro-
chromic supercapacitors and pseudocapacitors based on
polymers or transition metal oxides in the literatures (Table
S6). Additionally, the achieved highest energy density was
0.59 μWh cm−2 at a power density of 9.21 μW cm−2, which is
higher than the previously reported devices (Figure 7(e))
[45–47]. Meanwhile, the devices delivered a remarkable
volumetric capacitance of 69.46 mF cm−3 at a current density
of 0.57 mA cm−3 and an energy density of 0.03 mWh cm−3
with a power density of 0.51 mW cm−3. The cycling stability
was tested by GCD curve at 0.04 mA cm−2. A 90% capaci-
tance retention was acquired after 6,000 charge/discharge
cycles, showing an excellent cycling stability (Figure 7(f)).
Excellent charge/discharge cycle stability and multicolor
electrochromic properties make TPA-bpy the promising
candidate for electrochromic supercapacitors. Compared
with the reported viologen-based supercapacitors, the su-
percapacitors containing TPA-bpy has a wider voltage win-
dow and good stability (Table S7). Therefore, in the next
work, we will combine TPA-bpy with non-electrochromic
supercapacitor materials (e.g., conductive polymers, hydro-
xides or transition metal oxides) to produce flexible elec-
trochromic supercapacitors with high energy density. TPA-
bpy exhibits energy storage characteristics, mainly due to the
introduction of packing-disruptive and three-dimensional
triarylamine groups into 4,4′-bipyridine, which not only re-
sulted in the enhancement of solubility (Table S5) but also
avoided concentration quenching effectively due to π-π in-
teractions among TPA-bpy molecules [35–37].
hibited electrochemical storage properties superior to CZ-
bpy due to its more excellent solubility and film-formation
ability. Supercapacitors containing TPA-bpy exhibited a
wide voltage window (0–2.0 V), long discharge time
(230.3 s at 0.01 mA cm−2), and 90% capacitance retention
after 6,000 charge/discharge cycles. Meanwhile, the super-
capacitor’s charge and discharge status was accompanied by
a color change from purple to yellow. This work provided an
effective approach to fabricate multifunctional optoelec-
tronic devices. In the future, more efforts will be made to
combine TPA-bpy with transition metal oxides, hydroxides
or conductive polymers, which show non-electrochromic
and excellent energy storage properties, to obtain new ex-
cellent electrochromic supercapacitor materials.
Acknowledgements This work was supported by the National Funds for
Distinguished Young Scientists (61825503), the National Natural Science
Foundation of China (61775101, 61804082), the Priority Academic Pro-
gram Development of Jiangsu Higher Education Institutions (YX030003),
Natural Science Foundation of Jiangsu Higher Education Institutions
(18KJB430021), the China Postdoctoral Science Foundation Funded Project
(2018M642286), Jiangsu Planned Projects for Postdoctoral Research Funds
(2019K047A), Natural Science Foundation of Jiangsu Province of China
(BK20180760), and Science Foundation of Nanjing University of Posts and
Telecommunications (NY217142).
Conflict of interest The authors declare no conflict of interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
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